Chromosomal Evolution in Grasshopper Mice (Onychomys: Cricetidae)

Abstract

Diploid number was consistent at 48 in all specimens of Onychomys thus far studied, but the number of pairs of acrocentric elements varied from 0 in O. leucogaster to 6, 7, and 9 in O. torridus and to 10 in O. arenicola. Chromosomal variation in leucogaster, arenicola, and the 6 and 7 acrocentric cytotypes of torridus could be explained by additions (or deletions) of heterochromatic short arms. Onychomys leucogaster differed chromosomally from Peromyscus crinitus by an inversion in two pairs (9 and 19) plus heterochromatic short arm additions to all other pairs except 1, 22, and 23. Onychomys leucogaster differed from Peromyscus melanotis by inversions in chromosome pairs 2, 3, 6, 19, and 20 plus heterochromatic short arm additions in all other pairs except 1, 22, and 23. Onychomys leucogaster differed from Baiomys taylori by inversions in chromosome pairs 1, 9, 19, 22, and 23 plus heterochromatic short arm additions in all other pairs. These data are in agreement with the proposal that the primitive karyotype for the early peromyscines had few biarmed elements (probable biarmed pairs are 1, 22, 23, and possibly 9) with heterochromatin restricted to the centromere. Convergent events are needed to explain the distribution of heterochromatic short arms within species of Onychomys as well as within the peromyscine genera.